Basing modeling of a holder
One approach:

Make sure the cursor is centered SHIFT + “C”
Add a Plane
TAB into Edit Mode
Subdivide the Plane press “W” > Subdivide



Delete one half



Add a Mirror Modifier and check the box next to “Clipping”



Select all of the vertices and press “S” + “Y” (in top view) to scale on the y axis only.



Select the edge and extrude up by pressing “E” + “Z” and Left Mouse Click to accept 



Press “G” + “X” and grab the edge inward (I exaggerated this)



Place the cursor near the edge and press ALT + “R” to create a radius. You can adjust the radius in the operator panel “Before” you do anything else.



Press “E” + “X” and extrude to the center



Add a Solidify Modifier and adjust the thickness to taste